It is known to design point-to-multipoint access networks based on Passive Optical Networks (PONs). In a PON, a single head end is connected to a number of outstations via optical fibres connected in a tree-like arrangement using one or more passive distributive optical couplers to achieve optical fanout at the logical hub of the network. Typically, separate fibres and couplers are provided for downstream and upstream traffic. Alternatively, traffic in both directions can be combined onto a single fibre using separate wavelengths for upstream and downstream traffic, or by sending optical signals in opposite directions using the same nominal wavelength in each direction. Using this technique, use of optical and electronic equipment at the head end is shared amongst many outstations, often enabling a more economic overall network. Because of the shared nature of the system, each outstation can use only a proportion of the traffic capacity in each direction.
PONs have been studied in detail and standards exist for systems using this technology (for example Full Service Access Network (FSAN)). Existing PON systems generally use a medium access layer protocol based on a fixed length frame structure in which short timeslots are allocated to each outstation to carry downstream traffic using time division multiplexing (TDM). Upstream traffic is also sent in short packets, either according to a fixed schedule, or allocated dynamically on a frame by frame basis. The technique is referred to as Time Division Multiple Access (TDMA). Typically, to maximise efficiency, particularly when using short packets, each outstation adjusts its transmission time according to its distance from the head end in order that packets from multiple outstations arrive at the head end consecutively, without overlap but with minimum inter-packet gap. Measuring the time differential arising from different physical distances between the head end and the various outstations is normally performed by control functions at the head end. These control functions then inform individual outstations how to configure the upstream transmission times in order to achieve interleaving. This process is known as ‘ranging’ or ‘marshalling’.
A disadvantage of this approach is that the ranging process involves considerable complexity and custom designed integrated circuits are required at both the head end and the outstations in a practical system.
A further drawback is that because PON systems built according to FSAN principles normally use short packets for information transfer, transmission of data already formatted into larger packets requires these larger packets to be segmented for transport over the PON, adding further complexity.
The rapid rise in the use of the internet has increased the requirement to transmit packets carrying Internet Protocol (IP). At the transport layer, these packets already often use Ethernet technology, which allows packet sizes of up to around 1500 bytes. Networks carrying in IP traffic must support packets of up to at least 576 bytes, larger than can be carried unsegmented in an FSAN PON.